OBJECTIVE: We evaluated the incidence of vesicoureteral reflux in fetuses with prenatally detected isolated mild fetal hydronephrosis. METHODS: Fetuses with isolated mild fetal hydronephrosis (defined as a fetal renal pelvis anteroposterior diameter of > or = 4 and < 10 mm before 24 weeks' gestational age) were prospectively evaluated with postnatal renal ultrasound and voiding cystourethrography within the first few weeks after delivery. Infants were evaluated regardless of whether or not renal pelvic dilatation was seen on postnatal ultrasound examination. RESULTS: Forty cases of mild fetal hydronephrosis were identified from the 5,432 patients cared for at our institution from February 1996 to December 1998 (overall incidence: 1/136). Cases involving aneuploidy (n = 1) and inadequate follow-up (n = 5) were excluded from the investigation. One fetus with documented mild hydronephrosis early in gestation had spontaneous resolution and did not undergo postnatal evaluation. Of the remaining 33 infants, 32 underwent postnatal renal ultrasound examination and all had voiding cystourethrography. Vesicoureteral reflux was identified in five (15%) of the neonates. Eighty per cent (four out of five) of these infants were male. Resolution of vesicoureteral reflux occurred in 75% (three out of four) of the infants available for follow-up within 2 years of birth. CONCLUSIONS: Isolated mild fetal hydronephrosis is associated with vesicoureteral reflux on postnatal voiding cystourethrography.